Since moving picture data have extremely large information quantity, in the case where those information are recorded for a long time, means for recording video signals after undergone efficient encoding and for efficiently decoding those recorded signals when they are read out is indispensable. To meet with such requirement, several efficient encoding systems utilizing correlation in the video signal are proposed. As one of them, there is the MPEG (Moving Picture Experts Group) system.
In accordance with the MPEG system, difference between picture frames of a video signal is first taken by making use of the interframe correlation to thereby reduce redundancy in the time axis direction. Thereafter, processing such as Discrete Cosine Transform (DCT), etc. is carried out by making use of the line correlation to reduce redundancy in the spatial axis direction to thereby efficiently encode the video signal.
In this MPEG system, since in the case where only the difference signal between two frame pictures is transmitted when the interframe correlation is utilized, the original picture cannot be restored (reconstructed), there is used a method in which pictures of respective frames are caused to be any pictures of three kinds of pictures of the I picture (Intra Picture), P picture (Predictive Picture) and B picture (Bidirectional predictive Picture) to combine frame pictures of these three kinds of pictures to carry out compression encoding. The I picture is picture data compressed by only corresponding frame picture. The P picture is picture data compressed on the basis of corresponding frame picture and frame picture of I picture preceding (earlier) with respect to the corresponding frame picture and nearest thereto (in point of time). The B picture is picture data compressed on the basis of pictures of three frames in total of corresponding picture, and frame pictures of I picture and P picture respectively nearest thereto (in point of time) before and after the corresponding picture. At this time, the unit when picture data every frame are caused to undergo compression processing is called Group Of Pictures (GOP).
In this MPEG system, when the interframe predictive encoding is carried out, there are instances where such encoding is conducted in the state crossing over two groups of pictures.
In more practical sense, in the case where, e.g., the configuration of the group of pictures is as indicated by the configuration of the group of pictures GOP.sub.1, GOP.sub.2, GOP.sub.3 . . . shown in FIG. 1, the B picture which is the leading picture of each group of pictures is prepared by I picture subsequent (succeeding) thereto and P picture which is the last picture of the group of pictures earlier (preceding) by one. For this reason, in order to decode the leading B picture of a certain group of pictures, picture data of the group of pictures earlier (preceding) by one is required.
Meanwhile, as shown in FIG. 2, for example, when switching into compressed picture data of material (data) B is carried out at the editing point P in the course of transmitting groups of pictures GOP.sub.A1, GOP.sub.A2 of compressed picture data of material (data) A to transmit groups of pictures GOP.sub.B1, GOP.sub.B2 of compressed picture data of the material B in a manner continuous to the group of pictures GOP.sub.A2 of compressed picture data thereafter to decode the compressed picture data thus transmitted, decoding is carried out by using P.sub.8 picture from the group of pictures GOP.sub.A2 of the material A which has not any correlation in decoding B.sub.9 picture of the group of pictures GOP.sub.B1. Accordingly, the picture quality would be degraded to much degree.
With actual circumstances as described above in view, an object of this invention is to provide a compressed video signal editing apparatus, a compressed video signal editing method and a compressed video signal decoding apparatus which can decode, without degrading the picture quality, compressed picture data edited after undergone switching on the way.